The invention relates to an optical amplifier having at least a semiconductor body comprising a substrate of a first conductivity type and at least a semiconductor layer structure situated thereon consisting of at least a first passive layer of the first conductivity type, a second passive layer of the second, opposite conductivity type, and between the first and the second passive layer an active layer and a pn junction, amplification of electromagnetic radiation within a wavelength range taking place at a sufficiently high current strength in the forward direction through the pn junction within a strip-shaped amplification region of the active layer, which has a greater effective refractive index and a smaller bandgap for the radiation to be amplified than the first and second passive layers, which comprises a plurality of quantum well layers (QW layers for short) of a semiconductor material having a direct band transition and mutually separated by barrier layers of a different semiconductor material, and in which a portion of the (QW and barrier) layers which form part of the active layer, to be called first portion hereinafter, are under tensile stress, while the strip-shaped amplification region is bounded in longitudinal direction by end faces which serve as input and output surfaces for the radiation to be amplified and which are of low reflection, the second passive layer and the substrate being electrically connected to connection conductors.
Such optical amplifiers are frequently used in optical communication technology. Large distances must often be bridged and/or strongly branched-out networks must be used in optical telecommunication systems such as optical glass fiber systems. Often a weak or attenuated optical signal must then be regenerated once or several times in its path by an optical amplifier. Amplification of radiation takes place in the active layer in such an amplifier. This amplification has its maximum at a wavelength which depends on, among other factors, the choice of the semiconductor material of the active layer, the thickness of the QW layers, and the Fabry-Perot (FP) resonances, which in their turn are determined by the positions of the end faces. Because the end faces have low reflection, for example owing to the fact that they are coated with an anti-reflection layer, an optical amplifier of the travelling-wave type is obtained with a comparatively wide-band amplification profile which is determined by material amplification only. The use of an MQW active layer further has major advantages, such as a much higher saturation power, greater amplification bandwidth, an improved noise number, and a higher saturation gain.
Such an optical amplifier is known from the Japanese Patent Application JP-A-01/251685 (date of publication Jun. 10, 1989) which was laid open to public inspection and published in Patent Abstracts of Japan, vol. 14, Jan. 8, 1990, no. 2 (E-868), p. 72. The known optical amplifier is manufactured in the GaAs/InAlGaAs material system, employing a Multi Quantum Well (MQW) active layer in which the QW layers are under tensile stress. Due to the tensile stress, the amplification profile of TM-polarized radiation, which has its peak value at a different wavelength than the amplification profile of TE-polarized radiation, is raised at the detriment of the level of the amplification profile of TE-polarized radiation. For a (lattice matched) MQW active layer, the amplification profile of TE-polarized radiation is higher than that of TM-polarized radiation. Thus, introduction of tensile stress reduces the sensitivity to the polarization of the incoming radiation. At a certain point, i.e. at a tensile stress of about 0,6% (in this specification, a comma is used to denote a decimal point), both amplification profiles will be almost equally high. For the wavelength at the point of intersection of the two profiles, the amplifier will be insensitive to the polarization of the incoming radiation, while the amplification is near its peak value, i.e. near the peak value of both profiles. The rise of the amplification profile for TM-polarized radiation is connected with the influence of mechanical stress on the positions of the energy levels of the light holes (LH) and heavy holes (HH) in the valency band of the semiconductor material of the layers forming part of the active layer: tensile stress results in that the level of the light holes comes closer to that of the heavy holes, so that the TM-mode is less at a disadvantage.
A disadvantage of the known optical amplifier is that the amplification at the point of intersection is obtained at a relatively high current. The corresponding relatively high dissipation limits the lifetime and thus the performance of the known amplifier at the wavelength where it is insensitive to polarization.